Embodiments are directed to radiopaque implantable structures (e.g., stents) formed of cobalt-based alloys that comprise cobalt, chromium and one or more platinum group metals, refractory metals, precious metals, or combinations thereof. Platinum group metals include platinum, palladium, ruthenium, rhodium, osmium, and iridium. Refractory metals include zirconium, niobium, rhodium, molybdenum, hafnium, tantalum, tungsten, rhenium, and precious metals include silver and gold. In one embodiment, the one or more included platinum group or refractory metals substitute at least partially for nickel, such that the alloy exhibits reduced nickel content, or is substantially nickel free. The stents exhibit improved radiopacity as compared to similar alloys including greater amounts of nickel.